The present invention is directed to cemented carbide cutting inserts having chemical vapor deposited coatings thereon for increasing the wear resistance of the cutting inserts.
Use of chemical vapor deposited (CVD) coatings on a carbide cutting tool, such as WC-Co or WC-TiC-TaC-Co cutting tools to increase the wear resistance of such cutting tools is well known. The improved performance is a result of chemical stability, refractory characteristics, hardness and a low coefficient of friction inherent in such coatings.
TiN, TiC and Al.sub.2 O.sub.3 are examples of such coatings. Each of these coatings exhibits the above described properties in varying degrees and ranges such that no one coating, by itself, performs optimally over the wide range of cutting conditions employed by industry. For example, Al.sub.2 O.sub.3 coatings are superior to the other coatings at high cutting speeds where high temperatures are encountered, because of the very high chemical stability and low thermal conductivity which are properties of the ceramic. On the other hand, at very low speeds where metal buildup often causes tool failure, there are indications that TiN coatings are superior to others because of their low coefficient of friction. Further, the combination of hardness and chemical stability inherent in TiC makes it the optimum coating over a very broad range of intermediate speeds. Clearly, a cutting insert having the properties of two or more of such coatings would provide a highly useful tool capable of operating over a wide range of conditions.
A straightforward approach to the foregoing problem would be to provide a multi-layer coating on the cemented carbide cutting tool, the coating consisting of two or more of the above described coatings. However, the major difficulty in producing such a cutting tool is in obtaining sufficient adherency between coating layers, especially between the ceramic Al.sub.2 O.sub.3 and the other coatings.
Various prior art cutting tools employ adjacent layers of Al.sub.2 O.sub.3 and TiN or TiC on a cemented carbide substrate. Two such tools are disclosed in U.S. Pat. Nos. 3,837,896 and 3,955,038 both on Lindstrom et al. Disclosed therein are cutting tools comprised of a cemented carbide substrate and a thin coating layer of Al.sub.2 O.sub.3. A diffusion barrier layer is stated to be required between the Al.sub.2 O.sub.3 layer and the carbide substrate due to the harmful catalyzing effect in the formation and growth of the oxide layer due to Co and/or C in the carbide substrate. Such barrier layer may consist of nitrides or carbides of titanium.
Another insert disclosed in U.S. Pat. No. 4,150,195 to Tobioka et al employs a multi-layer coating deposited upon a carbide substrate. The multi-layer coating may include aluminum oxide as the most exterior layer, titanium carbonitride for the most interior coating layer, and titanium oxycarbonitride as an intermediate layer between the aluminum oxide and titanium carbonitride layers. The stated use of the intermediate layer of titanium oxycarbonitride is to increase the adhesive strength of the multi-layer coating.
None of the above cutting tools employ such multi-layer coatings for the purpose of providing the beneficial operating characteristics of each of the individual coating layers. That is, the prior art cutting inserts having an exterior layer of Al.sub.2 O.sub.3 are designed to provide the cutting characteristics of the Al.sub.2 O.sub.3 coated insert only, the underlying coating layers being provided merely as barriers between the oxide layer and the carbide substrate.
A novel coating procedure has now been discovered which allows the secure bonding of TiC, and/or TiN onto an Al.sub.2 O.sub.3 coated carbide cutting tool, thereby providing TiC and/or TiN as exterior coating surfaces on top of an Al.sub.2 O.sub.3 interior coating surface. Such a cutting tool exhibits the beneficial characteristics of TiC, TiN and Al.sub.2 O.sub.3 in combination.
In accordance with the invention, a thin titanium oxide layer is disposed between the ceramic and the TiN and/or TiC coating, the titanium oxide layer functioning to increase the adherency between the ceramic and other coatings.